haiku: fix Mesa build

1. The hgl.c file is a read-only file versus read-write.
Ref: src/gallium/state_trackers/hgl/hgl.c

2.  I've included the Haiku-specific patches I used to get a successful
build of Mesa 19.1.7 on Haiku using the meson/ninja build procedure.
Shows "[764/764] linking target ... libswpipe.so" at build completion.

v2:
Remove autotools files (Eric)

v3:
Update the patch

Reported-by: Ken Mays <kmays2000@gmail.com>
Tested-by: Ken Mays <kmays2000@gmail.com>
CC: mesa-stable@lists.freedesktop.org
Reviewed-by: Alexander von Gluck IV <kallisti5@unixzen.com>
(cherry picked from commit 4943c89d)